The present invention relates to a television system with access control using a variable electronic key.
The problem of access control of broadcast information mainly consists of the jamming of information on transmission by means of an appropriate key, as well as unjamming or deciphering on reception by means of an identical key, which is only in possession of users who have regularly paid a subscription fee.
Numerous television systems are already known which are equipped with jamming means on transmission and unjamming means on reception. Such systems are more particularly described in French Pat. No. 106 670, German Pat. No. 1 907 580, French Patent Application No. 75 34029 filed on Nov. 3, 1975 in the joint name of the French State and Telediffusion de France, French Pat. No. 1 034 776, German Pat. No. 1 254 676 and U.S. Pat. Nos. 2,510,046, 2,619,530, 2,892,882 and 2,972,009.
The jamming processes used in these systems are based on the introduction of a displacement of the position of the line synchronization pulse on a periodic reversal of the complete signal and on the polarity change of the video signal or on a combination of these operations. In all these processes it is necessary for the receivers to receive in some way or the other signals making it possible to restore the key necessary for covering the clear information.
The present invention relates to a television system of this type, but in which the key which permits unjamming is transmitted to the receivers in an original and advantageous manner.
In the prior art systems the transmission of this key does not give entire satisfaction because it is not generally compatible with the recently developed digital services and which are being or will be offered to viewers. They consist more particularly of three systems: a system for digital acquisition and television display of pictures organised in writing pages, a system for interactive terminal for teletext with call-in by numbering, and a system for programmed recording of transmissions on command from the sources.
The first system consists of a broadcast videotex system making it possible to insert alphanumeric information organized in pages and in magazines on television channels. The second comprises an interactive videotex system compatible with the first system and giving access to data bases (general information, lists, etc.) and to interactive services (transactions, messages, instruction) by the telephone system. The third system is a programme message system consisting of offering by means of a data broadcasting channel the remote control from a transmission source of the recording action by a receiver (e.g. a magnetoscope) of a preselected television programme.
For broadcasting information these three systems use means and a process of broadcasting of digital data, which is compatible with the broadcasting of the television signal.